This competing continuation proposal is aimed at understanding the molecular aspects of the pathogenesis of Legionnaire's disease, caused by the human pathogenic bacterium, Legionella pneumophila. Previous work by others has shown the importance of the intracellular life cycle in mononuclear phagocytic cells, particularly alveolar macrophages, in the pathogenesis of this disease, Up until now, through, no molecular basis for these cellular events has been identified. Recently the investigators, using the approach of site-specific mutagenesis, have isolated an isogenic mutant that is dramatically altered in its intracellular characteristics. This mutant, defective in the synthesis of a 24kDa protein antigen, is markedly attenuated in its ability to infect human alveolar macrophages, an observation first suggested by similar attenuation in a U937 cell model infection. The mutant is also defective in its ability to inhibit the oxidative burst of human polymorphonuclear leukocytes and its virulence is attenuated in an animal model of infection (i.e., intratracheally-inoculated guinea pigs). The DNA sequence of the gene encoding this 24kDa antigen indicates that the protein is a polycation, rich in lysine residues. Initial antibody binding data suggest the protein is surface located. Together these results suggest a number of possible mechanisms of action of the protein. In their future work the investigators propose to utilize a combination of approaches (cell biologic, biochemical, molecular genetic) to characterize the mechanism of the 24kDa effect on human cells. Specifically, they will determine the effects on phagocytic cells, will purify the protein to characterize its behavior, and will finemap functional domains within the molecule.